Method of etching the interior of a metal-containing glass switch



3,174,85 Patented Mar. 23, 1965 3,174,885 METHOD F ETCHING THE INTERIOR OF A METAL-CGNTAIN IN G GLASS SWITCH William G. Hyzer, Janesville, Wis., and Terry P. Smith,

Freeport, llll., assignors to Honeywell End, a corporation of Delaware N0 Drawing. Filed Nov. 17, 1961, Ser. No. 153,211 3 Claims. ((Il. 156-45) The present invention is directed to glass having a treated surface such that when a certain amount of liquid metal is associated with the treated surface the adhesion therebetween is less than the adhesion that arises when the same amount of the same liquid metal is associated with an untreated surface of like glass. The invention is further directed to methods of providing the treated surface.

A specific application in which low adhesion between the surface of glass and liquid metal is of particular importance is with respect to tilt operated glass mercury switches where it is desirable that regular or smooth movement between a globule of mercury and the interior surface of the glass envelope occurs upon a particular and/or minimum tilt taking place.

The prior art contains a number of expedients for reducing the adhesion between the globule of mercury and the interior surface of the glass envelope of a mercury switch but they all exhibit shortcomings. Thus, in the British Patent 11,281 of 1910 and the Walker Patent 1,803,986 organic substances such as creosote and alcohol, respectively, are suggested to be added to a globule of mercury to reduce sluggishness of movement of the glob ule of mercury upon a tilting of the mercury switch taking place. No doubt such practices are of help insofar as reducing sluggishness of movement of the globule of mercury is concerned but the utilization of organic sub stances in a mercury switch gives rise to the problem of contamination of the mercury which affects the electrical operation of the mercury switch. Another approach is directed to providing a roughened interior surface of the envelope by a chemical etch, a mechanical etch, or by sand blasting and these are set forth in the Mattern Patent 2,023,740, the Warren Patent 1,144,973, and the Baer et al. Patent 2,033,372, respectively. The roughened surface provided of course includes high and low surfaces in the forms of ridges and valleys. The mercury being noncapillary rests only on the ridges and not in the valleys. Because of the reduced surface area cont-act between the globule of mercury and the roughened surface, as compared to the same glob-ule of mercury and a like surface not roughened, the adhesion therebetween is reduced, again as compared to the same globule of mercury and the like surface not roughened. This approach while providing good result-s initially has a shortcoming in that the ridges tend to Wear down due to movement back and forth thereacross of the globule of mercury upon the repeated tilting of the mercury switch which takes place in use thereby resulting in an increase in surface area contact between the globule of mercury and the roughened surface and consequently results in an increase in the adhesion therebetween. A still further approach sug gested by the P-atnode Patent 2,306,222 is to apply a film of organosilicon halide to the interior surface of the envelope of a glass mercury switch so as to reduce adhesion between the surface and the gl-ob-ule of mercury. This approach as was the case with the approach just previously considered gives good results initially but the treated surface also is subject to wear in that erosion of the film arises due to the movement back and forth thereacross of the globule of mercury. This wear or erosion while affecting the adhesion between the surface and the globule of mercury thereby affecting the repetitive operating characteristics of the mercury switch insofar as tilt is concerned also introduces a contamination problem with respect to the mercury because of the organic character of the film which is eroded and which is absorbed by the mercury. A further shortcoming of at least the latter two approaches set forth above is that with the utilization of either of them it is difficult to produce uniform mercury switches, that is, switches which exhibit the same relative adhesion between the interior surface and the globule of mercury. This is because of the difiiculty of producing interior surfaces both roughened and with a film from envelope to envelope which are alike to the degree required. This of course means that it is very diflicult to control within exact limits the amount of tilt of the mercury switch necessary to cause regular and smooth movement of the globule of mercury.

In general, in the present invention a treated surface is provided on glass by selectively removing a constituent material, specifically an oxide of a polyvalent material from the surface of the glass by means of a solution including a so-called chelating agent. The treated surface thus provided not only is fast with respect to liquid metal, that is, the adhesion between the treated surface and the liquid metal is less than the adhesion between the untreated surface of like glass and the same amount of the same liquid metal but the shortcomings experienced with the expedients considered above are not encountered.

In the preferred embodiment of the present invention the interior surface of a lead glass mercury switch envelope is treated to remove lead oxide from the surface by exposing it to a solution containing a certain percentage of ethylene diamine tetra acetic acid :at a certain temperature and for a certain period of time. The percentage and temperature of the solution and the period of time are interrelated and are dependent upon the exact type of glass utilized and the treated surface or fastness desired. It will be appreciated that the problem of contamination of the mercury by organic substances does not arise because none are added to the mercury initially nor is any disposed on the surface treated. Further, it has been found that wear of the treated surface does not arise upon movement back and forth thereacross by the globule of mercury and this is thought to be the case because the removal of the lead oxide from the interior surface of the glass envelope not only provides the treated surface but a surface having a finite thickness, the make up of which approaches pure quartz which of course is very hard and not very susceptible to wear. Also since the invention is essentially directed to a chemical reaction, it can be monitored in any of a number of ways to assure control of the reaction so that the interior surface treated is uniform from envelope to envelope thereby helping to insure uniformity of operating characteristics from switch to switch.

Therefore, it is an object of the present invention to provide glass with a treated surface which is fast to liquid metal, which does not incorporate organic substances, which is not susceptible to wear, and which can be made uniform from surface to surface.

This and other objects will become more apparent from a reading of the following specification and appended claims.

The invention while having wide application will be set forth with respect to manufacture of tilt operated glass mercury switches.

A particular mercury switch to which the invention has been applied is one which utilizes a tubular envelope of a so-called G-l lead glass and particularly a 0010 type which is marketed by Corning Glass Works. This particular type of glass has approximately the following composition:

Percent Si 63 7 PhD 20.4

Na O 8.0 K 0 5.7 CaO 1.0

To obtain the proper adhesion in a mercury switch between the interior surface of the envelope and a globule of mercury so as to obtain regular or smooth movement of the globule of mercury upon a particular tilt occurring, in this case upon a range of angles of 0.4 to 1.2 being attained, a prior practice has been to apply a film of organosilicon halide by means of a vapor depositing method upon the interior surface of the envelope which at this stage is open at one extremity and which supports an electrode or electrodes dependent upon the particular design. The depositing of the film is followed by a chemical cleaning operation which is provided to remove any organic substances which may have been deposited upon the electrode or electrodes, as the case may be, by the vapor depositing method. With a globule of mercury deposited in the envelope and heat applied to the open extremity, the envelope can be sealed by being tipped or pinched off.

It has been observed through actual experience and life testing that of such switches which are initially satisfactory ultimately 10% of them fail in that they exceed the 1.2 high angle limit, that is, a tilt exceeding 1.2 is required to cause smooth or regular movement of the globule of mercury. This is brought about because of the wearing or eroding of the film by the movement back and forth thereacross of the globule of mercury which results in a change of the adhesion therebetween. Of course wearing or eroding away of the film by the globule of mercury introduces organic substances into the mercury. Also because of the heat applied to the tipped or pinched off area to allow sealing of the envelope, the film in this area is vaporized and destroyed. This has two effects. One effect is to provide a further source of organic substances to contaminate the mercury and the other is to destroy the relative uniformity of the interior surface of the envelope which can result in changes in the regularity or smoothness of movement of the globule of mercury. As to the contamination of the mercury by the introduction of organic substances, it should be noted this results in an increase in electrical resistance of the switch. Further, since the film is deposited by a vapor depositing method, it is extremely difficult to control the film thickness and hence the uniformity of the treated surface from envelope to envelope.

As applied to an envelope of 0010 type glass described above, the present invention is practiced as follows. The envelope which is open at one extremity and which supports an electrode or electrodes, dependent upon the particular design, has disposed therein or is immersed in a bath so as to have disposed therein an aqueous solution including 4 to 5% by weight of the chelating agent, ethylene diamine tetra acetic acid which results in a solution having a pH of 13.1 to 13.2, and a trace of a wetting agent. The wetting agent is added for the usual purpose of assuring proper contact between the solution and the surface of the envelope to be treated. The interior surface of the envelope is maintained in contact with the solution which is kept at approximately 205 F. for a period of approximately hours. During this time, a chemical reaction takes place whereby lead oxide is removed from the surface by the chelating agent. The resulting treated surface to the naked eye is no different from that of an untreated surface of the same glass. However, the treated surface does include a microscopic etch. And as mentioned previously, the treated surface is of a finite thickness and has a chemical make up approaching pure quartz. While the solution utilized to provide the treated surface contains organic substances, these are water soluble and'hence any such substances that might remain in the envelope and/ or on the electrode or electrodes, as the case may be, after the solution has been removed from the envelope can easily be washed away by means of a water rinse. A globule of mercury comparable to that deposited in the envelope including the film provided by the organosilicon halide is deposited in the envelope under consideration and is sealed as set forth previously.

Because the treated surface has a make up approaching pure quartz, wear or erosion of the surface due to movement back and forth thereacross of the globule of mercury is indiscernible. Through actual experience and life testing, it has been observed that such switches never exceed the 12 high angle limit. Further, since quartz is so very hard, the heat applied to the tipped or pinched off area to allow sealing of the envelope has no effect on the treated surface in this area and consequently the treated surface is uniform throughout assuring no changes in the regularity or smoothness of the movement of the globule of mercury. The absence of organic substances assures that contamination of the mercury does not arise. It has been determined that because of this the electrical resistance of such switches is 4% lower than for comparable switches which include the film provided by the organosilicon halide. Also since the surface is treated by means of chemical reaction uniformity can be assured. Further, uniformity from surface to surface of envelopes is possible. Thus by monitoring the conductivity of the solution and/ or the optical density of the solution and/ or the pH of the solution, for example, control of the chemical reaction can be maintained and hence close tolerances can be maintained with respect to the surface and from surface to surface.

As pointed out previously, the percentage of ethylene diamine tetra acetic acid in and the temperature of the solution and the time of reaction are interrelated. For example, the reaction can be accomplished with the same results obtaining at ordinary room temperature but of course the time required is increased quite considerably. On the other hand, by doubling the percentage of ethylene diamine tetra acetic acid in and maintaining the 205 F. temperature of the solution, the time required for the reaction to produce the same results is approximately cut in half. Further, the desired surface will dictate the variables as will the type of glass utilized.

While the chelating agent utilized in the preferred embodiment is ethylene diamine tetra acetic acid, a chelating agent such as nitrilo tri acetic acid and tri sodium salt mono hydrate can be utilized in the solution so as to provide the surface on the glass of the general type under consideration.

Further, while the liquid metal where it has been mentioned has been mercury, it will be appreciated that the surface provided by the invention could be utilized with other liquid metals such as mercury thallium.

Also while the invention in the preferred embodiment is directed to a mercury switch application, there are other applications for the invention. For example, in mercury thermometers, the utilization of the present invention on the interior surface would reduce the tendency for the mercury to remain separated as often happens after shock has been applied to .such a device.

In view of the foregoing, therefore, the scope of the present invention should be determined from the following claims in which we claim:

1. The method of treating the interior surface of a tubular lead glass housing open at one end so as to make the interior surface fast to mercury and particularly adapted to serve as a mercury switch envelope, comprising: filling the housing with an aqueous solution including approximately 4 to 5% by weight of ethylene diamine tetra acetic acid providing a pH of approximately 13.1 to 13.2 and exposing the housing containing the forementioned solution to a temperature of approximately 205 F. for a period of approximately 10 hours, the ethylene diamine tetra acetic acid acting to remove lead oxide from the interior surface.

2. The method of producing a mercury switch having a tubular lead glass housing comprising: exposing the interior surface of an open ended envelope to an aqueous solution including ethylene diamine tetra acetic acid to remove lead oxide from the surface of the glass, the percentage of ethylene diarnine tetra acetic acid in the solution and the temperature of the solution along with the time period of the surface of the glass is exposed to the solution being interrelated and chosen so as to provide the surface of the glass with the desired fastness to mercury, introducing a globule of mercury into the envelope and sealing the envelope.

3. The method of producing a liquid metal switch having a tubular housing formed of glass having a polyvalent metallic oxide as a constituent comprising: exposing the interior surface of an open ended envelope to a solution including a chelating agent of the class capable of re moving the polyvalent oxide from the interior surface, the percentage of chelating agent in the solution and the temperature of the solution along with the time period the surface of the glass is exposed to the solution being interrelated and chosen so as to provide the surface of the glass with the desired fastness to liquid metal, introducing a globule of liquid metal into the envelope and sealing the envelope.

References Cited by the Examiner UNITED STATES PATENTS 1/56 Parker 156-24 XR OTHER REFERENCES EARL M. BERGERT, Primary Examiner.

JACOB H. STEINBERG, Examiner. 

1. THE METHOD OF TREATING THE INTERIOR SURFACE OF A TUBULAR LEAD GLASS HOUSING OPEN AT ONE END SO AS TO MAKE THE INTERIOR SURFACE "FAST" TO MERCURY AND PARTICULARLY ADAPTED TO SERVE AS A MERCURY SWITCH ENVELOPE, COMPRISING: FILLING THE HOUSING WITH AN AQUEOUS SOLUTION INCLUDING APPROXIMATELY 4 TO 5% BY WEIGHT OF ETHYLENE DIAMINE TETRA ACETIC ACID PROVIDING A PH OF APPROXIMATELY 13.1 TO 13.2 AND EXPOSING THE HOUSING CONTAINING THE FOREMENTIONED SOLUTION TO A TEMPERATURE OF APPROXIMATELY 205*F. FOR A PERIOD OF APPROXIMATELY 10 HOURS, THE ETHYLENE DIAMINE TETRA ACETIC ACID ACTING TO REMOVE LEAD OXIDE FROM THE INTERIOR SURFACE. 